Relating topography and soil phosphorus distribution in litter‐amended pastures in Arkansas

Abstract

Abstract Poultry producers in northwestern Arkansas fertilize pastures with litter, leading to excessive P buildup on surface soils with risk of contaminating nearby surface waters. Information on the influence of pasture topography on P runoff is limited. Objectives were to assess soil P and P index status in pastures, quantify topographic influence on P distribution, and generate high‐resolution P maps for site‐specific nutrient management. Soil samples were collected from a commercial farm in a grid design and analyzed for Mehlich‐3 P (STP), and dissolved reactive P (DRP). Gburek (GPI) and Sims P indices (SPI) were calculated by considering soil erosion and runoff potentials, STP, and P fertilizer application rate and source. A machine‐learning algorithm, based on a random forest model, quantified spatial relationships of STP, DRP, and P indices with topography. The study area was highly variable in topography and soil P levels. High‐slope areas bordering a stream and flat areas with lower elevation had greater GPI and SPI. Topography explained up to 50% of variation in STP and DRP distribution and >70% variation in GPI and SPI, which was linked to litter application sites and cattle feeding sites. Key terrain attributes for STP, DRP, GPI, and SPI distribution were elevation, slope position, slope height, valley depth, and valley bottom flatness. Predicted P maps illustrated areas along a stream had lower STP and DRP levels, but greater GPI and SPI. This analysis linked topographic relationships with P distribution, as topography controls the flow and distribution of water and influences farm management practices; therefore, future P management strategies should explicitly incorporate topographic risks.